Interceptor that exposes the current MethodInvocation
as a thread-local object. We occasionally need to do this; for example, when a pointcut
(e.g. an AspectJ expression pointcut) needs to know the full invocation context.

Don't use this interceptor unless this is really necessary. Target objects should
not normally know about Spring AOP, as this creates a dependency on Spring API.
Target objects should be plain POJOs as far as possible.

If used, this interceptor will normally be the first in the interceptor chain.